1. Field of the Invention
The present invention relates to a perforating device used by a stamp unit for perforating a desired character string pattern on a print face portion of the stamp unit that incorporates an ink impregnation member, the underside of which is covered with heat sensitive stencil paper including a thermoplastic film. More particularly, the invention relates to a perforating device that prevents dregs of melted thermoplastic film from getting stuck to the heating elements of the thermal head when the thermoplastic film in the heat sensitive stencil paper is thermally perforated, whereby the heating elements are kept clean to constantly perforate unblurred character string patterns.
2. Description of Related Art
Perforating devices are known that utilize heating elements of a thermal head to thermally perforate a desired character string pattern on heat sensitive stencil paper including a thermoplastic film.
One such perforating device is disclosed in Japanese Patent Laid-Open No. Sho 62-70079, the subject matter of which is incorporated herein by reference. This device includes dot heating elements that heat a desired pattern in order to thermally perforate the thermoplastic film into dots constituting a thermally perforated portion of the film. While the dots are being made by heat, the thermoplastic film slidingly contacts the heating elements. After the dots are complete, ink is applied through the thermally perforated portion to form an image pattern on a recording medium. According to this recording method, the thermoplastic film slidingly contacts the dot-type heating elements as they are moved relative to each other to form the thermally perforated portion where the perforated holes are made even in diameter. This apparently allows the device to provide clear recording images.
The recording method disclosed in Japanese Patent Laid-Open No. Sho 62-70079, the subject matter of which is incorporated herein by reference, discusses the thermoplastic film slidingly contacting the heating elements as they are moved relative to each other to form the thermally perforated portion. This causes dregs of the melted plastic resin to accumulate at one end of the thermally perforated portion. Because the perforations are made instantaneously, the heating elements are generally heated to a temperature sufficiently higher than the melting temperature of the thermoplastic film. This causes dregs of the melted plastic resin to attach to the heating elements. As the heating elements rapidly cool, the melted plastic resin attached to the elements is likely to solidify thereon.
As a result, the heating elements remain soiled. The plastic resin is difficult to remove from the heating elements and therefore may lead to blunting of thermally made dot patterns. This results in significantly lower quality images.
To prevent the deterioration of the dot pattern requires keeping the surface of the heating elements clean. One such cleaning method is described in Japanese Patent Laid-Open No.. Hei 2-204076, the subject matter of which is incorporated herein by reference. This method involves having a cleaning material of thin fiber press against and slid along the thermal head while the thermal head is being heated.
However, the cleaning material used by the above cleaning method is only for cleaning the thermal head. That is, the material is not designed to clean the thermal head while thermal perforation is in operation. This cleaning material is periodically applied (independent of thermal perforation) after a predetermined amount (time) of perforations is carried out. In other words, thermal perforation cannot be performed while the thermal head is being cleaned. In addition, cleaning by the material takes place only upon completion of thermal perforation. If dregs of the melted plastic resin attach to the thermal head during the thermal perforation, the perforating operation must nevertheless proceed. This can entail the blunting of perforated patterns by the plastic resin that is melted and stuck. As a result, the cleaning method fails to eliminate degrading of the quality of dot pattern images formed by perforated film.